Very early in the discovery of the biologically remarkable and structurally unique peptides from the sea hare Dolabella auricularia, which have been designated dolastatins, it became clear that certain members (e.g. 10-15) exhibited a variety of important properties that include anticancer (Pettit, G. R. In International Oncology Updates: Marine anticancer compounds in the era of targeted therapies; Chabner, B.; Cortés-Funes, H., Eds.; Permanyer Publications: Barcelona, 2009; Kingston, D. G. I. J. Nat. Prod. 2009, 72, 507-515; Singh, R.; Sharma, M.; Joshi, P.; Rawat, D. S. Anti-Cancer Agents Med. Chem. 2008, 8, 603-617; Pettit, G. R. In Progress in the Chemistry of Organic Natural Products; Herz, W.; Kirby, G. W.; Moore, R. E.; Steglich, W.; Tamm, C., Eds.; Springer: Vienna, 1997; Vol. 70, 1-79) and antifungal activities (Woyke, T.; Roberson, R. W.; Pettit, G. R.; Winkelmann, G.; Pettit, R. K. Antimicrob. Agents Chemother. 2002, 46, 3802-3808; Woyke, T.; Berens, M. E.; Hoelzinger, D. B.; Pettit, G. R.; Winkelmann, G.; Pettit, R. K Antimicrob. Agents Chemother. 2004, 48, 561-567). Indeed, dolastatin 10 and three structural modifications are currently in human cancer phase II and phase III clinical trials (Pettit, G. R. In International Oncology Updates: Marine anticancer compounds in the era of targeted therapies; Chabner, B.; Cortés-Funes, H., Eds.; Permanyer Publications: Barcelona, 2009). Two derivatives of dolastatin 15 are also in cancer clinical trials (phase I-II) (Pettit, G. R. In International Oncology Updates: Marine anticancer compounds in the era of targeted therapies; Chabner, B.; Cortés-Funes, H., Eds.; Permanyer Publications: Barcelona, 2009).
When the present inventors extended the field collections of D. auricularia from the Indian Ocean to the Western Pacific (Papua New Guinea and the Philippines), the dolastatin series was expanded to dolastatin 16-19 (Pettit, G. R.; Xu, J.-p.; Hogan, F.; Williams, M. D.; Doubek, D. L.; Schmidt, J. M.; Cerny, R. L.; Boyd, M. R. J. Nat. Prod. 1997, 60, 752-754; Paterson, I.; Findlay, A. D. Aust. J. Chem. 2009, 62, 624-638; Paterson, I.; Findlay, A. D. Pure Appl. Chem. 2008, 80, 1773-1782). Dolastatin 16, shown below as compound 1, especially proved to be an exceptionally potent inhibitor of cancer cell growth and a candidate for further development (Pettit, G. R.; Xu, J.-p.; Hogan, F.; Williams, M. D.; Doubek, D. L.; Schmidt, J. M.; Cerny, R. L; Boyd, M. R. J. Nat. Prod. 1997, 60, 752-754).
Structurally, dolastatin 16 is a cyclodepsipeptide containing two new amino acids, dolamethylleuine (Dml, 2), a β-amino acid, and dolaphenvaline (Dpv, 3). The structure of dolastatin 16 without assignment of the configuration of the novel amino acids was achieved by high-field NMR and tandem MS/MS mass spectroscopic interpretations (Pettit, G. R.; Xu, J.-p.; Hogan, F.; Williams, M. D.; Doubek, D. L.; Schmidt, J. M.; Cerny, R. L.; Boyd, M. R. J. Nat. Prod. 1997, 60, 752-754).
Development of dolastatin 16 as an anticancer agent has been delayed for two reasons. First, dolastatin 16 was originally isolated in low yield (in 3.1×10−7% yield) as an amorphous powder. Second, long period of attempts at crystallization were unsuccessful. Thus, to develop dolastatin 16, there is a need for unequivocal configurational assignments and a practical total synthesis for scale-up production.
Other options for obtaining certain dolastatin members appeared likely some 35 years ago when considering the fact that Dolabella species derived nutrition by consuming marine microalgae and that such exogenous sources might provide the dolastatins or intermediates (Pettit, G. R. In Progress in the Chemistry of Organic Natural Products; Herz, W.; Kirby, G. W.; Moore, R. E.; Steglich, W.; Tamm, C., Eds.; Springer: Vienna, 1997; Vol. 70, 1-79). This expectation has been amply realized over the past decade by the isolation of dolastatins 10-16 or close analogues from the cyanobacterium Lyngbya majuscula and other such microalgae (Pettit, G. R.; Xu, J.-p.; Hogan, F.; Williams, M. D.; Doubek, D. L.; Schmidt, J. M.; Cerny, R. L.; Boyd, M. R. J. Nat. Prod. 1997, 60, 752-754; Luesch, H.; Moore, R. E.; Paul, V. J.; Mooberry, S. L.; Corbett, T. H. J. Nat. Prod. 2001, 64, 907-910; Harrigan G. G.; Yoshida, W. Y.; Moore, R. E.; Nagle, D. G.; Park, P. U.; Biggs, J.; Paul, V. J.; Mooberry, S. L.; Corbett, T. H.; Valeriote, F. A. J. Nat. Prod. 1998, 61, 1221-1225; Nogle, L. M.; Williamson, R. T.; Genvick, W. H. J. Nat. Prod. 2001, 64, 716-719; Gunasekera, S. P.; Miller, M. W.; Kwan, J. C.; Luesch, H.; Paul, V. J. J. Nat. Prod. 2010, 73, 459-462; Adams, B.; Pörzgen, P.; Pittman, E.; Yoshida, W. Y.; Westenburg, H. E.; Horgen, F. D. J. Nat. Prod. 2008, 71, 750-754; Taori, K.; Liu, Y.; Paul, V. J.; Luesch, H. Chem Bio Chem 2009, 10, 1634-1639; Nogle, L. M.; Genvick, W. H. J. Nat. Prod. 2002, 65, 21-24). Thus, fermentation methods using marine cyanobacteria may eventually be competitive with total syntheses for scale-up production of new anticancer drugs in the family. At present, the yields from these initial experiments remain very low, and for the foreseeable future the provision of dolastatin 16 for cancer clinical trial development will require a practical total synthesis for scale-up production.